1. Field of the Invention
The present invention relates to a cardiac monitoring device of the type having a component for transmitting a signal and receiving at least one echo of the signal reflected from at least one cardiac segment, a position of which, at least when reflecting the signal is related to the cardiac performance of a heart, and circuitry to determine a delay between the transmission of the signal and the receipt of the echo, and to derive from the delay the position of the cardiac segment.
The invention also relates to a rate responsive pacemaker system containing such a device.
2. Description of the Prior Art
Since a device of the above type is particularly suited for an application wherein it is arranged to register the position of a cardiac wall segment, it will, by way of example only, be described in relation to such an application hereinafter. Thereby, the cardiac performance refers to cardiac output and/or other parameters related to cardiac frequency, such as beat volume, cardiac contractility etc.
A device of the above type is described in U.S. Pat. No. 3,938,502, which describes a device for examining a hollow organ, such as a heart, having a catheter which is to be placed inside the hollow organ and which is provided at its end with circumferentially arranged, equidistantly distributed elements, each of which serves both for the transmission and reception of ultrasonic waves. In order to make it possible to completely visualize the moving cardiac structure, such as heart walls and heart valves, surrounding the catheter, at any moment, the device presents a large number of elements which show very little directivity in a plane perpendicular to the axis of the catheter, the elements being so dimensioned in the axial direction that the major part of acoustic energy in transmission is confined to a plane perpendicular to the axis of the catheter. Additionally, this known device includes an excitation unit which successively excites groups of adjacently arranged elements at a rate of at least 25 times per second. The transmitted and the received pulses for the elements of a group are respectively delayed so that the differences in travel times among the elements for the pulses to or from a line in the plane perpendicular to the catheter axis and being perpendicular to the center line of the group are compensated. This known device further includes an adder for the summation of the echo pulses brought into coincidence by the time delays. The known device is used for visually displaying the part of the examined hollow organ surrounding the catheter. This patent teaches the use of the equipment for direct imaging by means of a so called brightness mode (B-mode) ultrasound. The catheter described is primarily intended for examining a visualizing a heart, but is obviously not meant for implantation and would not possibly be used for such a purpose as it is considered highly energy consuming and would be of unacceptable size and weight for operation as an implantable equipment.
European Application 0 503 839 discloses a method and an apparatus for chronologically monitoring the hemodynamic state of a patient using Doppler ultrasound. Such a method and apparatus are used for regulating blood flow within the cardiovascular system in a closed-loop control system using ultrasound measurement techniques to determine a hemodynamic status of a patient and to derive a control parameter for modulating the hemodynamics of the system using electrical or pharmaceutical therapy. Heart contractility and the blood flow output from the heart is monitored in order to control an implantable cardiac assist or therapy device to maintain cardiac output without invading the left heart or the arterial system of the patient, Thus, the pacing rate of a cardiac pacemaker may be based on the determination of the cardiac output estimated by this device. The device is arranged to measure the cardiac output using Doppler ultrasound techniques in which a measuring transducer is implanted within the right heart and directed towards the left ventricle or aortic root. The transducer radiates acoustic energy at ultrasonic frequencies, then the device determines blood flow velocity by receiving and processing the resulting echo signals and measuring the shift in frequency of the returning echoes in comparison to the transmitted waves. The integral of the mean velocity curve is an accurate representation of stroke volume and cardiac output. However, the device is not provided to register the delay between the transmission of the signals and the receipt of the echoes thereof in order to derive from the delay a position of a certain cardiac segment. Moreover, the Doppler equipment to be used in the above application requires quite a lot of energy for its operation, and the output acoustic energy is likely to be too high to be used for permanent operation in a human body.
However, a device as disclosed in European Application 0 503 839 offers a possibility to register the hemodynamic situation in the systemic high pressure circulation of a heart, that is in the left atrium and left ventricle thereof, without needing to be located in said high pressure system. This is an advantage in comparison to prior pacemaker systems that are developed primarily to detect physiologic changes in the right atrium and/or right ventricle even though the action of the pulmonary or right side of the heart is only minimally representative of the hemodynamic situation in the high pressure part of the heart system,
According to modern research and scientific publications as by Baan et at (Baan J. Jong I T. Kerkhof P L. Moene R i. van Dijk A D. van der Velde E I. Koops J. Continuos stroke volume and cardiac output from intra-ventricular dimensions obtained with impedance catheter. Cardiovascular Research 1981 June;15(6):328-34), a very good correlation exists between left ventricular diameter and cardiac output performance. The changes in intra-ventricular dimensions have been measured by means of electrical impedance. For this purpose, a catheter has been equipped with a number of electrodes spaced over a distance equal to the long axis of the left ventricle into which the catheter has been introduced. A constant current was imposed between the outermost electrodes while the inner ones were used to measure resistance of volume segments of the blood contained within the ventricular cavity. The difference in resistance at the beginning and the end of ejection was proportional to the contribution of each segment to stroke volume. The best correlation of left ventricular diameter to cardiac output performance was obtained when measuring in the middle of the axial length of the left ventricle.
An object of the present invention is to provide a cardiac monitoring device which permits to estimate the total cardiac performance of a heart, including the performance of the high pressure side of said heart, without interfering the function of the heart or being easily interfered by the function of the heart. Moreover, the device should be of a type that is implantable in mammals, and particularly human beings, and which permits to be used in such an application for a significant period of time. The device should also be of a type that requires a minimum of power consumption and is simple as to its construction.
The above object is achieved in accordance with the principles of the present invention in an implantable cardiac monitoring device having an A-mode ultrasound probe which is adapted to be positioned in the right ventricle at a heart, and which emits an ultrasound signal which is reflected from at least one cardiac segment of the left ventricle of the heart, the ultrasound probe receiving the resulting echo signal, and circuitry for determining a delay between the emission of the ultrasound signal and the reception of the resulting echo, and for determining, from this delay, a position of the cardiac segment, the position of this cardiac segment, at least when reflecting the signal, being related to cardiac performance, and the implantable cardiac monitoring device including a unit for deriving, from the detected position, the cardiac performance.
By directing the transmitted and correspondingly received signal or signals towards a cardiac segment the position of which correlates very well to the cardiac performance, that is the cardiac output performance, the device may be used to provide a pacemaker with information concerning that performance. Such information may then be used by the pacemaker system in order to suitably control the very pacing of the heart. The inventive device permits monitoring the condition of the left, high pressure side of the heart, while being positioned at another location, for instance in the pulmonary side of the heart.
As noted above, in a preferred embodiment of the device, the device is arranged to be positioned in at least one of the right atrium and right ventricle of a heart. Thereby, left atrial or ventricular movements and dimensions may be detected in a minimally invasive way while avoiding those problems that may appear if the device would be located in the left part of the heart, that is the left atrium or ventricle.
According to another preferred embodiment of the device, the ultrasound probe is arranged to direct the ultrasound signal toward the left ventricular wall of a heart, this wall defining the aforementioned cardiac segment. The echoes of the signal thus are received from the left ventricular wall and the position of the left ventricular wall at a certain movement during a cardiac cycle is correlated to the inner diameter of the left ventricle at that specific movement or, at least, to the volume of the left ventricle of the heart, the diameter or volume being correlated, in turn, to the cardiac output performance. Thus, by deriving the position of the left ventricular wall, a precise estimation of the momentary cardiac performance may be obtained.
According to another preferred embodiment of the device, the ultrasound probe is arranged to direct emitted ultrasound signal toward the medial portion and lateral portion of the endocardial wall of the left ventricle respectively, these portions defining the aforementioned cardiac segment, and the device includes circuitry which determines the echo delay difference between the echoes respectively reflected from these portions, and which, from this difference, determines the distance between these wall portions, this distance corresponding to the cardiac performance. Such an arrangement is particularly advantageous as it has been shown that the distance between these portions correlates very well with the cardiac output. In order to achieve this, the device is preferably arranged at a catheter or the like arranged to be inserted in or near the heart and anchored therein in quite a fixed position.
According to another preferred embodiment, the inventive device forms a part of a rate responsive pacemaker system and is arranged to provide said system with cardiac performance information. The pacing may be adapted to the cardiac performance in every single situation. For example, in a DDDR-mode, the atrium and the ventricle operate in synchrony. At high atrial rates this synchronous behavior may be inappropriate or harmful. The pacemaker, by means of the inventive device, detects when synchronous behavior is harmful by measuring the cardiac output. While an atrial rate causes an elevation in cardiac output, the pacemaker allows the ventricle to respond to the atrial rate. When further increases in atrial rate lead to a sustained degradation in cardiac output, the pacemaker no longer allows synchronous pacing. By measuring cardiac output, the pacemaker system is able to determine when ventricular activity should not follow natural atrial heart beats and is able to adjust to a suitable pacing mode. In addition, using an indication of cardiac output, the pacemaker system can determine whether the heart is successfully responding to a pacing stimulus of a particular amplitude and pulse duration.
The ultrasound probe emits ultrasound according to the A-mode principle, defining the emitted signal. Thereby, the power consumption is minimal due to the fact that the periodic measurement frequency may be low, for example twice per cardiac cycle or less, and that, ideally, transmission may be performed in one direction only.
In one embodiment of the device, the ultrasound probe is formed by at least one ultrasound crystal, arranged on a pacemaker electrode. By arranging the crystal on the electrode a very precise position of the crystal may be defined. For example, a tip at the end of the electrode is arranged to be anchored at the very bottom of the right ventricle, resulting in the electrode extending through the ventricle in a very well defined way, thereby making it possible to locate the crystal with good precision at a suitable location along the electrode.
In another embodiment of the device the ultrasound probe is formed by an array of ultrasound crystals, distributed around the periphery of a pacemaker electrode, each of the ultrasound crystals being arranged to individually transmit and receive ultrasound pulses in a co-ordinated manner. This arrangement is to assure optimal transmission and reception performance even at possible rotation of the pacemaker electrode at the time of implantation or later. By checking which crystal receives the most appropriate echo, transformation and receipt of the ultrasound signals may then be performed with that single crystal. There is always at least one crystal active with optimal measurement performance and directed in a correct direction, e.g. toward the site of the left ventricle where the wall movements correspond best to the volume changes thereof and are large enough to be easily detected. For example, the crystals of the array may be arranged to scan their detection area at predetermined or optional occasions and the device can include circuitry to survey the scanning results to identify the best measured echo performance in order to choose which crystal or crystals are to be used for transmission until the next scanning event. By only receiving echoes within a certain temporal window and studying the amplitudes of the echoes received by the respective crystal within said temporal windows, it is possible to determine which echo performance is the best and most suitable one for further measurement. Preferably, the device is connected to or contains a logic unit of a pacemaker system, said logic unit being arranged to treat the information from each individual crystal and to determine which echo performance is the best.
According to another version of the embodiment wherein the ultrasound probe has a single ultrasound crystal, the device has a circuit for triggering the emission of ultrasound by single crystal at desired moments of a cardiac cycle from sensed IEGM-information by a pacemaker electrode connected this circuit. Thereby, the device is able to trigger two consecutive transmissions at selected moments during one or two cardiac cycles, e.g. when the ventricular volume is supposed to be at or near its minimum and maximum respectively, in order to derive the positions of the aforementioned cardiac segment at these moments in order to estimate the respective volumes at those moments, corresponding to the cardiac performance. By performing the transmission and receipt at such moments a very good correlation between the position of the respective cardiac segment, for instance the left ventricular walls, and the cardiac output is obtained. Particularly, the difference between minimum and maximum inner diameters of the left ventricle has a good correlation to the cardiac output, said correlation being taken advantage of by this specific arrangement of the device.
Another object of the present invention is to provide a rate responsive pacemaker system capable of estimating the cardiac performance, that is a cardiac output, and to use this information as a parameter in order to control its pacing or other operation.
Such a rate responsive pacemaker system contains a monitoring device according to the invention as previously described.